In a rectifier, such as a rectifier that sees use in an AC-to-DC converter, there is power loss. When current is flowing through the rectifier, there is a forward voltage drop across the rectifier. If, for example, this rectifier is typical diode full bridge rectifier, then on the first phase of a cycle, an incoming AC signal current flows through two of the diodes of the bridge. There is a forward voltage drop across each of these diodes. The forward voltage drop at peak current flow may, for example, be about one volt. Then, in the second phase of the cycle of the incoming AC signal, current flows through the other two of the diodes of the bridge. Again, there is a forward voltage drop across each of these diodes. The voltage drop across each of these diodes may be about one volt at the time of peak current flow. The product of the instantaneous current flow through such a rectifier, multiplied by the instantaneous voltage drop across the rectifier, is the instantaneous power that is lost in the rectifier. Over one cycle of the incoming AC signal, the average power lost is the integral of the instantaneous voltage drop across the rectifier multiplied by the instantaneous current flow through the rectifier, divided by the period of the cycle. Reducing this average power loss is desirable.
Circuits and techniques have been proposed to reduce such power loss. In one type of circuit, the rectifier includes a field effect transistor. A voltage detector circuit detects the voltage between the source and drain of the field effect transistor. If the voltage is negative, then the voltage detector supplies a signal onto the gate of the field effect transistor such that the transistor is controlled to be off. If, however, the voltage is detected to be positive, then the voltage detector supplies a signal onto the gate of the field effect transistor such that the transistor is controlled to be on. The overall circuit can therefore be considered to be a synthetic rectifier or a synthetic diode. The overall circuit acts like a diode in that it conducts current between source and drain if there is a positive voltage across the device, whereas it does not conduct current if there is a negative voltage across the device. If the forward voltage drop across the device when it is conducting current is less than the forward voltage drop that would otherwise occur across a diode, then using the synthetic diode circuit in place of a diode in a rectifier can serve to reduce power loss. The article entitled “Diode Simulator Reduces Forward Drop To 0.04V”, EDN Magazine, page 212, by Isaac Eng (Jul. 20, 1992) sets for one example of such a circuit. In another type of circuit, a bipolar transistor and a parallel-connected diode are used in place of a rectifier diode. If the voltage across the bipolar transistor device is negative, then associated circuitry controls the bipolar transistor to be off. Current does not flow through the bipolar transistor, and the diode is reverse biased so current does not flow through the parallel-connected diode. If, however, there is current flow due to a forward voltage condition, then the associated circuitry supplies a base current to the bipolar transistor such that the bipolar transistor is controlled to turn on. The forward voltage drop across the bipolar transistor and the parallel-connected diode when the bipolar transistor is controlled to be on in this way can be as small as 0.1 volts. U.S. Pat. No. 8,649,199, entitled “Low Forward Voltage Rectifier”, filed on Oct. 29, 2011, issued on Feb. 11, 2014, by Kyoung Wook Seok, sets forth an example of such a low forward voltage rectifier circuit. There are various advantages and drawbacks of these two types of circuits.